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Cement chemist notation

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Title: Cement chemist notation  
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Subject: Cement, Pozzolanic reaction, Concrete, CCN, Tricalcium aluminate
Collection: Cement, Chemical Formulas, Concrete, Notation, Oxide Minerals, Silicates
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Cement chemist notation

Cement chemist notation (CCN) was developed to simplify the formulas cement chemists use on a daily basis. It is a shorthand way of writing the chemical formula of oxides of calcium, silicon, and various metals.

Contents

  • Abbreviations of oxides 1
  • Conversion of hydroxides in oxide and free water 2
  • Main phases in Portland cement before and after hydration 3
    • Clinker and non-hydrated Portland cement 3.1
    • Hydrated cement paste 3.2
  • Use in ceramics, glass, and oxide chemistry 4
  • Possible use of CCN in mineralogy 5
  • See also 6
  • References 7
  • External links 8

Abbreviations of oxides

The main oxides present in cement (or in glass and ceramics) are abbreviated in the following way:

CCN Actual Formula Name
C CaO Calcium oxide, or lime
S SiO2 Silicon dioxide, or silica
A Al2O3 Aluminium oxide, or alumina
F Fe2O3 Iron oxide, or rust
T TiO2 Titanium dioxide, or titania
M MgO Magnesium oxide, or periclase
K K2O Potassium oxide
N Na2O Sodium oxide
H H2O Water
C CO2 Carbon dioxide
S SO3 Sulfur trioxide
P P2O5 Phosphorus hemi-pentoxide

Conversion of hydroxides in oxide and free water

For the sake of mass balance calculations, hydroxides present in hydrated phases found in hardened cement paste, such as in portlandite, Ca(OH)2, must first be converted into oxide and water.

To better understand the conversion process of hydroxide anions in oxide and water, it is necessary to consider the autoprotolysis of the hydroxyl anions; it implies a proton exchange between two OH, like in a classical acid-base reaction:

OH + OH → O2− + H2O
acid 1 + base 2 → base 1 + acid 2

or also,

2 OH → O2− + H2O

For portlandite this gives thus the following mass balance:

Ca(OH)2 → CaO + H2O

Thus portlandite can be written as CaO • H2O or CH.

Main phases in Portland cement before and after hydration

These oxides are used to build more complex compounds. The main crystalline phases described hereafter are related respectively to the composition of:

  • Clinker and non-hydrated Portland cement, and;
  • Hardened cement pastes obtained after hydration and cement setting.

Clinker and non-hydrated Portland cement

Four main phases are present in the clinker and in the non-hydrated Portland cement.
They are formed at high temperature (1 450 °C) in the cement kiln and are the following:

CCN Actual Formula Name Mineral Phase
C3S 3 CaO • SiO2 Tricalcium silicate Alite
C2S 2 CaO • SiO2 Dicalcium silicate Belite
C3A 3 CaO • Al2O3 Tricalcium aluminate Aluminate or Celite
C4AF 4 CaO • Al2O3 • Fe2O3 Tetracalcium alumino ferrite Ferrite

The four compounds referred as C3S, C2S, C3A and C4AF are known as the main crystalline phases of Portland cement. The phase composition of a particular cement can be quantified through a complex set of calculation known as the Bogue Formula.

Hydrated cement paste

Hydration products formed in hardened cement pastes (also known as HCPs) are more complicated, because many of these products have nearly the same formula and some are solid solutions with overlapping formulas. Some examples are given below:

CCN Actual Formula Name or Mineral Phase
CH Ca(OH)2 or CaO • H2O Calcium hydroxide
C-S-H 0.6-2.0(CaO)•SiO2•0.9-2.5(H2O), with variable composition within this range, and often also incorporating partial substitution of Al for Si Calcium silicate hydrate
C-A-H This is even more complex than C-S-H Calcium aluminate hydrate
AFt C3AS3H30-32, sometimes with substitution of Fe for Al, and/or CO32- for SO42- calcium trisulfoaluminate hydrate, or ettringite
AFm C2ASH12, often with substitution of Fe for Al, and/or various other anions such as OH- or CO32- for SO42- Calcium monosulfoaluminate
C3AH6 3CaO • Al2O3 • 6 H2O Hydrogarnet

The hyphens in C-S-H indicate a calcium silicate hydrate phase of variable composition, while 'CSH' would indicate a calcium silicate phase, CaH2SiO4.

Use in ceramics, glass, and oxide chemistry

The cement chemist notation is not restricted to cement applications but is in fact a more general notation of oxide chemistry applicable to other domains than cement chemistry sensu stricto.

For instance, in ceramics applications, the kaolinite formula can also be written in terms of oxides, thus the corresponding formula for kaolinite,

Al2Si2O5(OH)4,

is

Al2O3 • 2SiO2 • 2H2O

or in CCN

AS2H2.

Possible use of CCN in mineralogy

Although not a very developed practice in mineralogy, some chemical reactions involving silicate and oxide in the melt or in hydrothermal systems, and silicate weathering processes could also be successfully described by applying the cement chemist notation to silicate mineralogy.

An example could be the formal comparison of belite hydration and forsterite serpentinisation dealing both with the hydration of two structurally similar earth -alkaline silicates, Ca2SiO4 and Mg2SiO4, respectively.

Calcium system: belite hydration:

Belite + water → C-S-H phase + portlandite
2 Ca2SiO4 + 4 H2O → 3 CaO · 2 SiO2 · 3 H2O + Ca(OH)2
2 C2S + 4 H → C3S2H3 + CH

Magnesium system: forsterite serpentinisation:

Forsterite + water → serpentine + brucite
2 Mg2SiO4 + 3 H2O → Mg3Si2O5(OH)4 + Mg(OH)2
2 M2S + 3 H → M3S2H2 + MH

The ratio Ca/Si (C/S) and Mg/Si (M/S) decrease from 2 for the di-calcium and di-magnesium silicate reagents to 1.5 for the hydrated silicate products of the hydration reaction. In other term, the C-S-H or the serpentine are less rich in Ca and Mg respectively. This is why the reaction leads to the elimination of the excess of portlandite (Ca(OH)2) and brucite (Mg(OH)2), respectively, out of the silicate system, giving rise to the crystallization of both hydroxides as separate phases.

The rapid reaction of belite hydration in the setting of cement is formally "chemically analogue" to the slow natural hydration of forsterite (the magnesium end-member of olivine) leading to the formation of serpentine and brucite in nature. However, the kinetic of hydration of poorly crystallized artificial belite is much swifter than the slow conversion/weathering of well crystallized Mg-olivine under natural conditions.

This comparison suggests that mineralogists could probably also benefit from the concise formalism of the cement chemist notation in their works.

See also

References

External links

  • Cement and Concrete Glossary
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